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Former 49ers great Dwight Clark announced Sunday that he has been diagnosed with ALS, or amyotrophic lateral sclerosis, which is a degenerative nerve disease that affects “nerve cells in the brain and spinal cord.” It’s also commonly known as “Lou Gehrig’s disease.”

Hours later, the foundation for Steve Gleason, the former Saints player who has battled ALS since 2011 and was the subject of a documentary about his fight against the disease, had this message for Clark:

Clark says he first became concerned something might be wrong when he felt “weakness in my left hand” in September 2015. And the man best known for “The Catch” is concerned that football may have played a part in his diagnosis.

“I’ve been asked if playing football caused this. I don’t know for sure. But I certainly suspect it did,” Clark wrote. “And I encourage the NFLPA and the NFL to continue working together in their efforts to make the game of football safer, especially as it relates to head trauma.”

Meanwhile, Gleason has lived with ALS for six years. This message is currently pinned at the top of his Twitter page: